1. Field of the Invention
The present invention relates to an EL (electro-luminescence) display formed by preparing an EL element on a substrate. More particularly, the invention relates to an EL display using a semiconductor element (an element using a semiconductor thin film). Furthermore, the present invention relates to an electronic device in which the EL display is used in a display portion thereof. The EL devices referred to in this specification may includes triplet-based light emission devices and/or singlet-based light emission devices.
2. Description of the Related Art
In recent years, technology for forming a TFT on a substrate has been largely improved, and an application development of the TFT to an active matrix type display device has been carried out. In particular, the TFT using a polysilicon film has a higher electric field effect mobility than the TFT using a conventional amorphous silicon film, and therefore, the TFT may be operated at a high speed. Thus, the pixel control which has been conducted at a driver circuit outside of the substrate may be conducted at the driver circuit which is formed on the same substrate as the pixel.
Such an active matrix type display device can, by preparing various circuits and elements on the same substrate, obtain various advantages such as a decrease in the manufacturing cost, a decrease in the size of the display device, an increase in the yield, and a decrease in the throughput.
Further, research on the active matrix type EL display having an EL element as a self-light-emitting device (hereafter referred to as an EL display) is becoming more and more active. The EL display is referred to as an organic EL display (OELD) or an organic light-emitting diode (OLED).
The EL display is a self-light-emitting type unlike a liquid crystal display device. The EL element is constituted in such a manner that an EL layer is sandwiched between a pair of electrodes. However, the EL layer normally has a lamination structure. Typically, the lamination structure of a xe2x80x9cpositive hole transport layer/a luminous layer/an electron transport layerxe2x80x9d proposed by Tang et al. of the Eastman Kodak Company can be cited. This structure has a very high light-emitting efficiency, and this structure is adopted in almost all the EL displays which are currently subjected to research and development.
In addition, the structure may be such that on the pixel electrode, a positive hole injection layer/a positive hole transport layer/a luminous layer/an electron transport layer, or a positive hole injection layer/a positive hole transport layer/a luminous layer/an electron transport layer/an electron injection layer may be laminated in order. Phosphorescent dye or the like may be doped into the luminous layer.
In this specification, all the layers provided between the pixel electrode and an opposite electrode are generally referred to as EL layers. Consequently, the positive hole injection layer, the positive hole transport layer, the luminous layer, the electron transport layer, the electron injection layer and the like are all included in the EL layers.
Then, a predetermined voltage is applied to the EL layer having the above structure from the pair of the electrodes, so that a recombination of carriers is generated in the luminous layer and light is emitted. Incidentally, in this specification, the fact that the EL element emits a light is described as the fact that the EL element is driven. Furthermore, in this specification, the light-emitting element formed of the anode, the EL layer and the cathode is referred to as an EL element.
Conventionally, the pixel structure of an active matrix type EL display device has generally been like that shown in FIG. 18. Reference numeral 1701 in FIG. 18 denotes a TFT functioning as a switching element (hereafter referred to as a switching TFT), reference numeral 1702 denotes a TFT functioning as an element for controlling the electric current supplied to an EL element 1703 (hereafter referred to as an EL driver TFT), reference numeral 1703 denotes the EL element, and reference numeral 1704 denotes a capacitor (storage capacitor).
Gate signal lines (G1 to Gy) for inputting gate signals are connected to gate electrodes of the switching TFTs 1701 of each pixel. Further, one set of regions of source regions and drain regions of the switching TFTs 1701 of each pixel are connected to source signal lines, also referred to as data signal lines (S1 to Sx) for inputting digital data signals, and the other set of regions is connected to gate electrodes of the EL driver TFTs 1702 of each pixel and to the capacitors 1704 of each pixel, respectively. Note that the digital data signal refers to a digital video signal.
One of the source regions of the EL driver TFTs 1702 of each pixel is connected to one of electric power supply lines (V1 to Vx), and the drain region is connected to the EL element 1703. The electric potential of the electric power supply lines (V1 to Vx) is referred to as an electric power supply potential. Further, the electric power supply lines (V1 to Vx) are connected to the capacitor 1704 of each pixel.
The EL element 1703 is composed of an anode, a cathode, and an EL layer formed between the anode and the cathode. When the anode is connected to the drain region of the EL driver TFT 1702, namely when the anode is a pixel electrode, the cathode becomes an opposing electrode. Conversely, when the cathode is connected to the drain region of the EL driver TFT 1702, namely when the cathode is the pixel electrode, the anode becomes the opposing electrode. The electric potential of the opposing electrode is referred to as an opposing electric potential throughout this specification. The electric potential difference between the electric potential of the opposing electrode and the electric potential of the pixel electrode is an EL driver voltage, and the EL driver voltage is applied to the EL layer.
A conventional method of driving an EL display is explained next. First, all of the switching TFTs 1701 having their gate electrode connected to the signal line G1 turn on in accordance with a gate signal input to the gate signal line G1. Note that the fact that all of the switching TFTs having their gate electrode connected to the signal line turn on in accordance with the gate signal is referred to as a gate signal line selection in this specification.
The digital data signal is then input into the source signal lines (S1 to Sx) in order. The opposing electric potential is maintained at the same level as the electric power supply potential of the electric power supply lines (V1 to Vx). The digital data signal has xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d information, and the xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d digital data signals specify signals having either high or low voltage.
The digital data signal input to the source signal lines (S1 to Sx) is then input to the gate electrode of the EL driver TFT 1702 through the on-state switching TFT 1701. Further, the digital data signal is also input to the capacitor 1704 and stored.
The gate signal lines G2 to Gy are then selected in order in accordance with the gate signal, and the above operations are repeated. Note that the input of the digital data signal to the gate electrode of the EL driver TFT through the switching TFT is referred to as inputting the digital data signal to the pixel throughout this specification. A period until the digital data signal is input to all of the pixels is referred to as a write-in period.
When the digital data signal is input to all of the pixels, all of the switching TFTs 1701 are turned off. The opposing electric potential is then given an electric potential difference to the electric power supply potential at a level in which the EL elements emit light. The digital data signal stored in the capacitor 1704 is then input to the gate electrode of the EL driver TFT 1702.
When the digital data signal has xe2x80x9c0xe2x80x9d information, the EL driver TFT 1702 is set to the off state and the EL element 1703 does not emit light. Conversely, when the digital data signal contains xe2x80x9c1xe2x80x9d information, the EL driver TFT 1702 turns on. As a result, the pixel electrode of the EL element 1703 is maintained at the electric power supply potential, and the EL element 1703 emits light. Thus in accordance with the information of the digital data signal, the selection of whether the EL element emits light or does not emit light is made and display is performed for all pixels at the same time. By performing display of all of the pixels, an image is formed. A period in which the pixels perform display is referred to as a display period.
The digital data signal is thus input to all of the pixels in the digital drive EL display during the write-in period. The input digital data signal is then stored in each pixel, and when the write-in period is complete, the display period begins and all of the pixels perform display at the same time.
With the above driver method, a time difference develops for the storage of the digital data signal between the pixel into which the digital data signal is first written into, and the last pixel into which the digital data signal is written into, within the write-in period.
The digital data signal is ideally stored as an electric charge in the gate electrode of the EL driver TFT when the switching TFT is in the off state. However, in practice, the electric charge gradually decreases due to a leak current caused by an off current of the switching TFT (a drain current which flows even when the TFT, as a switch, is in the off state). The reduction in the electric charge due to the leak current occurs easier the longer the electric charge storage time becomes. Therefore, the nearer the beginning of the write-in period that the digital signal is written into the pixel, the more that the electric charge stored in the gate electrode of the EL driver TFT will tend to decrease.
It is necessary to store the electric charge of the EL driver TFT gate electrode from when the digital data signal is input in the write-in period until the display period is complete. If the electric charge stored in the gate electrode of the EL driver TFT decreases, then the brightness of light emitted by the EL element will fall, and a desired gradation will not be obtained. Even if a digital data signal for preforming display having the same brightness is input to each pixel, display having the same brightness is not obtained for the first pixel into which the digital data signal is written and for the last pixel into which the digital data signal is written.
By connecting the gate electrode of the EL driver TFT to a storage capacitor, it is possible to supplement, to a certain degree, the electric charge of the gate electrode which decreases due to the leak current. However, the electric charge which accumulates in the storage capacitor is also reduced by the leak current. There are times, therefore, when the reduction in electric charge stored in the EL driver TFT gate electrode is not sufficiently supplemented, and the brightness of the light emitted by the EL elements is reduced.
An object of the present invention is to provide means for resolving the above types of problems. Namely, an object of the present invention is to prevent a reduction of an electric charge, stored in a gate electrode of an EL driver TFT, by a leak current of a switching TFT, and to prevent a decrease in the brightness of light emitted by an EL element.
As means for fulfilling the above objects, a volatile memory SRAM is formed in the present invention between the gate electrode of the EL driver TFT and, from among a source region and a drain region of the switching TFT, the region which is not connected to a source signal line. Differing from a DRAM (dynamic random access memory), the SRAM (static random access memory) is not limited by removing an electric power supply, but rather stores input data until the next data is input. Further, compared to the DRAM, the amount of time needed to input the data is shorter with the SRAM, and it is possible to perform high speed write-in of data.
It becomes possible to store a digital data signal input to a pixel during a write-in period, until a display period is complete, with the above structure. In other words, it becomes possible to prevent the electric charge stored in the gate electrode of the EL driver TFT from being reduced by the leak current of the switching TFT, and it becomes possible to prevent the brightness of the light emitted by the EL element from falling.
Note that it is possible to form the volatile memory using a TFT, and therefore it is possible to form the volatile memory at the same time as the switching TFT and the EL driver TFT.
Note that a storage capacitor need not be actively formed in the present invention. If the storage capacitor is not formed, it becomes possible to shorten the amount of time for inputting the digital data signal to the pixels. Therefore, even if there is an increase in the number of EL display pixels, the amount of write-in time can be controlled.
The structure of the present invention is shown below.
According to the present invention, there is provided an electro-optical device comprising a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric power supply lines, and a plurality of pixels, characterized in that:
the plurality of pixels are each composed of a switching TFT, an SRAM, an EL driver TFT, and an EL element;
one of a source region and a drain region of the switching TFT is connected to one of the plurality of source signal lines, and one of the source region and the drain region of the switching TFT is connected to an input side of the SRAM, respectively;
an output side of the SRAM and a gate electrode of the EL driver TFT are connected;
the source region of the EL driver TFT is connected to one of the plurality of electric power supply lines, and the drain region of the EL driver TFT is connected to a cathode or an anode of the EL element, respectively; and
the SRAM stores a digital data signal input to the SRAM from one of the plurality of source signal lines, through the switching TFT, until the next digital data signal is input to the SRAM.
According to the present invention, there is provided an electro-optical device comprising a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric power supply lines, and a plurality of pixels, characterized in that:
the plurality of pixels are each composed of a switching TFT, an SRAM, an EL driver TFT, and an EL element;
one of a source region and a drain region of the switching TFT is connected to one of the plurality of source signal lines, and one of the source region and the drain region of the switching TFT is connected to an input side of the SRAM, respectively;
an output side of the SRAM and a gate electrode of the EL driver TFT are connected;
the source region of the EL driver TFT is connected to one of the plurality of electric power supply lines, and the drain region of the EL driver TFT is connected to a cathode or an anode of the EL element;
a period within one frame period during which the EL element emits light is controlled by using a digital data signal; and
the SRAM stores the digital data signal input to the SRAM from one of the plurality of source signal lines, through the switching TFT, until the next digital data signal is input to the SRAM.
According to the present invention, there is provided an electro-optical device comprising a plurality of source signal lines, a plurality of gate signal lines, a plurality of electric power supply lines, and a plurality of pixels, characterized in that:
the plurality of pixels are each composed of a switching TFT, an SRAM, an EL driver TFT, and an EL element;
one of a source region and a drain region of the switching TFT is connected to one of the plurality of source signal lines, and one of the source region and the drain region of the switching TFT is connected to an input side of the SRAM;
an output side of the SRAM and a gate electrode of the EL driver TFT are connected;
the source region of the EL driver TFT is connected to one of the plurality of electric power supply lines, and the drain region of the EL driver TFT is connected to a cathode or an anode of the EL element;
one frame period is divided into n sub-frame periods SF1, SF2, . . . , SFn;
the n sub-frame periods SF1, SF2, . . . , SFn have write-in periods Ta1, Ta2, . . . , Tan, and display periods Ts1, Ts2, . . . , Tsn, respectively;
a digital data signal is input to all of the plurality of pixels during the write-in periods Ta1, Ta2, . . . , Tan;
whether the plurality of EL elements emit light or do not emit light during the display periods Ts1, Ts2, . . . , Tsn is selected in accordance with the digital data signal;
a ratio of the length of the display periods Ts1, Ts2, . . . , Tsn is expressed by 2( ) :: 2xe2x88x921 :: . . . 2xe2x88x92(n-1); and
the SRAM stores the digital data signal input to the SRAM from one of the plurality of source signal lines, through the switching TFT, until the next digital data signal is input to the SRAM.
The present invention may have a characteristic in that the SRAM has two n-channel TFTs and two p-channel TFTs.
The present invention may have a characteristic in that:
source regions of the two p-channel TFTs of the SRAM are connected to a high voltage side of an electric power supply, and source regions of the two n-channel TFTs are connected to a low voltage side of the electric power supply;
one p-channel TFT and one n-channel TFT form a pair;
the drain regions of the p-channel TFT and n-channel TFT pairs are mutually connected;
the gate electrodes of the p-channel TFT and n-channel TFT pairs are mutually connected;
the drain regions of one p-channel TFT and n-channel TFT pair are maintained at the same electric potential as that of the gate electrodes of the other p-channel TFT and n-channel TFT pair; and
the drain regions of one p-channel TFT and n-channel TFT pair are an input side for inputting the digital data signal, and the drain regions of the other p-channel TFT and n-channel TFT pair are an output side for outputting a signal in which the polarity of the input digital data signal is inverted.
The present invention may have a characteristic in that the SRAM has two n-channel TFTs and two resistors.
The present invention may have a characteristic in that:
drain regions of the two n-channel TFTs of the SRAM are connected to a high voltage side of an electric power supply, and source regions of the two n-channel TFTs of the SRAM are connected to a low voltage side of the electric power supply through one of the two resistors;
the drain regions each of the two n-channel TFTs are mutually maintained at the same electric potential as a gate electrode of the other n-channel TFT; and
from among the two n-channel TFTs, the drain region of one n-channel TFT is an input side for inputting the digital signal, and the drain region of the other n-channel TFT is an output side for outputting a signal in which the polarity of the input digital data signal is inverted.
The present invention may have a characteristic in that:
the plurality of EL elements have an EL layer between the anode and the cathode; and
the EL layer is a low molecular weight organic material or an organic polymer material.
The present invention may have a characteristic in that the low molecular weight material is composed of Alq3 (tris-8-quinolinolate aluminum) or TPD (triphenylamine dielectric).
The present invention may have a characteristic in that the organic polymer material is composed of PPV (polyphenylene vinylene), PVK (polyvinyl carbazole), or polycarbonate.
The first frame period may be equal to or less than {fraction (1/60)} second.
The present invention may be a computer, a video, or a DVD, which is characterized by using the electro-optical device.